The present invention relates to a magnetic disk storage device, wherein a magnetic servo-head and a magnetic data-head are disposed in a back to back relationship within a space between two magnetic disks. In particular, it relates to a magnetic shield means for shielding a magnetic servo-head from noise generated by an adjacent magnetic data-head during a write operation.
Magnetic disk storage devices are widely used as a non-volatile storage means because of their compact size and high storage capacity. At present, the storage capacity 51/4 inch storage devices is, for instance, 400 mega bytes. In such a device, a plurality of magnetic disks (hereinafter "disks") are stacked in parallel and co-axially on a rotatable spindle, and spaced apart from each other by a predetermined distance so as to form a block or a disk assembly. The spindle is rotated by a driving motor at a high speed such as 3600 rpm.
A magnetic disk comprises a layer of a substrate made of, for example, aluminum. Both sides of the substrate are coated with a magnetic medium, such as .gamma.-ferric oxide (.gamma.Fe.sub.2 0.sub.3) particles. Information is stored in the magnetic layer in the form of data bits sequentially arranged in concentric circles referred to as a tracks. The data bits are created in the magnetic medium of a track by a magnetic data-head (hereinafter "data-head"). The data-head is actuated in response to data signals supplied from a central controller of the storage device. This is refereed to as a write operation, and the region on the surface of the disk where the data bits are formed is referred to as a data region. The data bits are accessed and read by the data head during a read operation period. Servo information, including address signals and servo signals, are stored in the form of servo bits in a servo region which usually occupies a part of the surface of a disk. The servo bits are read by a magnetic servo-head (hereinafter "servo-head").
Usually, a magnetic disk storage device has a plurality of data-heads and only one servo-head. Generally, each of these comprises an electromagnetic transducer and an air slider. The transducer and air slider are flexibly suspended from a suspending member which includes a gimbal spring and a spring arm. The suspending member is secured to a rigid supporting member at an end of the suspending member opposite the end where the transducer is mounted. The suspending member keeps the transducer at a predetermined flying height, such as 0.2 .mu.m, over the surface of a rotating disk by balancing the weight of the transducer, air slider, suspending member and rigid supporting member against the aerodynamic lift produced by the air slider. New data is written to and read from the data region via the transducer and a magnetic field in the gap between the transducer and the disk.
The supporting members are stacked tightly around a spindle. The stacked arrangement of supporting members is referred to as a head block. The spindle is supported by a supporting means, such as ball bearings, so as to be rotatable around an axis of the spindle and to be driven by a voice coil motor (VCM) attached to the head block. The VCM is controlled by access signals typically provided by a central computer. The head block is rotated clockwise or counter-clockwise so that the data-heads move along an arc. The data-heads therefore move approximately perpendicular to each track. To access the disk storage device, track address signals and track error signals are read by the servo-head and fed back to an access control circuit which forms a closed loop feed back control system. Movement of the data-heads when accessing a track is performed under the control of access signals.
The above-described access mechanism for moving data-heads to a specified track is well-known and referred to as a swinging head positioner. There is also a head positioner of another type which is movable along a straight line running in a radial direction of the disk. This is referred to as a linear head positioner, and uses a carriage on which the head block is mounted. With this mechanism heads can traverse the tracks exactly perpendicularly.
The need for high speed read/write operations, high recording densities and high reliability, results in a high degree of configurational precision and results in a number of problems to be solved. One of the problems is erroneous operation of the servo-system of the disk device during a write operation of a nearby data-head. This error is induced by electro-magnetic noise caused because the data-head transducer. The transducer used in the data and servo heads includes a small coil. During a write operation of a data-head, data signals (e.g., a series of pulse trains) are sent from the central computer and applied to the coil. This is called actuating the data-head to produce data bits on the relevant track. But, the pulse signals induce magnetic leakage flux to emanate from the coil. The frequency of the noise ranges from a low of, e.g., 1.5 MHZ, to a high of, e.g., 2 MHZ. If a servo-head is located near the data-head, the servo-head detects the leakage flux; thus, causing noise in the servo-head. The induced noise adversely affects the servo-operation of the disk storage device. Despite the noise problems, positioning a data-head and a servo-head in the above-described manner is common.
Generally, in recent disk storage devices each side of the disk is used as a storage medium. Thus, two magnetic heads are placed in the space between adjacent stacked disks, each head facing a respective disk surface. The heads are inserted in these spaces via a head positioner mechanism.
It is desirable to place the servo-head in a centrally located one of the spaces between the stacked disks of a disk assembly. The reason for is as follows. The actual axis of rotation of a disk assembly tends to be subject to a slight inclination off the ideal position due to unavoidable tolerances in the relevant supporting structure for supporting and rotating the spindle. If the servo-head is placed in a centrally located space, the off-track positioning error of the data-heads located near the end portions of the stacked disk assembly is minimized. This is because the disk assembly is positioned with no off-track error at the center portion of the disk assembly since the servo-head is centrally positioned. In such a configuration, a servo-head is always placed near a data-head, and is therefore subject to noise from the data-head. Because of the noise, this configuration is not desirable.
An existing noise reduction scheme is described below. A magnetic disk assembly having a servo-head that is surrounded by an electromagnetic shield cap. Unexamined Japanese Patent Application, Ser. No. 60-136024, published on July 19, 1985, by Matsumoto discloses a magnetic disk assembly having a servo-head that is surrounding by an electromagnetic shield cap. This arrangement attaches the shield cap to the associated supporting arm as described below.
FIG. 1 is a schematic side view illustrating the arrangement of disks 2 to 5 stacked co-axially around a rotatable spindle 1. As shown in FIG. 1, magnetic heads 6 to 13 are respectively mounted on the tips of suspending members 14 to 21. Gimbal springs and spring arms are fixed to supporting arms 22 to 26. As seen from the figure, the heads are arranged on the same cylinder; namely, a cylindrical plane that is co-axial with the spindle 1. Although it is not shown in FIG. 1, the stacked supporting arms 22 to 26 comprise a head assembly of a swinging head positioner. The head 10 is a servo-head and the other heads are data-heads. Each surface of the disks 2 to 5 is used as a data region, except for a part of an upper surface 4a of the disk 4. The region 4a is used to store servo information. The heads face the disk surfaces, and are at a predetermined flying height above the surfaces when the disk assembly is rotated at operational speeds. As shown in FIG. 1, the servo-head 10 and the data-head 9 are arranged in a back-to-back configuration within the same cylinder. Due to the close proximity of the heads 9 and 10, the servo-head 10 is subject to the noise generated by the data-head 9 during a write operation.
In order to reduce the undesirable effect of the noise, a shield cap 27 of magnetic material is disposed on a tip portion 24a extended from the supporting arm 24. FIG. 2 is a perspective view of the shield cap 27 which comprises a top wall 27a and a U-shaped side wall 27b, defining a space for housing the servo-head 10. Thus, the servo-head 10 is electro-magnetically shielded from noise generated by the data-head 9 during write operations. However, adding the shield cap 27 and the extended portion 24a to the supporting arm 24, undesirably increases weight and the moment of inertia, about the axis of the spindle 1, of the head assembly. This results in various serious disadvantages such as increased access time and erroneous head positioning. In addition, an undesirable mechanical resonance of the servo-head 24 tends to occur during access operations.
Another shielding means is disclosed in unexamined Japanese Patent Application, Ser. No. 60-140524, published on July 25, 1985, by Seki. This application discloses a shield plate fixed to a supporting arm and extending in a plane in which the supporting arm lies. The shield plate is interposed between a data-head and a servo-head, both of which are fixed to the supporting arm parallel to each other and extend in the longitudinal direction of the supporting arm. By this configuration, the servo-head is shielded from noise generated by the data-head. However, the increase in the weight of movable members of the head assembly causes a disadvantages similar to that of the device proposed by Matsumoto.
In order to overcome such problems unexamined Japanese Patent Application, Ser. No. 58-17515, published on Feb. 1, 1983, by Sengoku discloses a shield plate that is secured to a base portion of the relevant housing of the device, and not to a moving member. In this case, a servo-head is disposed to face the bottom surface of the bottom disk, which is the case with a magnetic disk device of rather low storage density. During normal operation, the servo-head is shielded from the noise generated by the other data-heads, that do not require a shield means. This is because the substrate of the disks is usually made of aluminum which has a high electrical conductivity and serves to shield the servo-head from noise, particularly high frequency noise. However, when a head, positioned facing the disk surface opposite the surface faced by the servo-head, accesses tracks near the peripheral edge of the disk, noise generated by the data-head leaks around the edge of the disk and adversely affects the servo-head. As stated above, shield plate of Sengoku is secured to a base portion of the relevant housing that accommodates the relevant disk assembly and head positioner. The shield has a horizontal shield wall disposed in the same plane as the bottom disk that is faced by the servo-head. The shield wall has concaved arc shaped edge that is positioned close to the disk edge, leaving a small gap between the shield edge and the disk edge. The shield plate is made of a magnetic material or electrically conductive material. Thus, magnetic leakage flux from the data-head facing the upper surface of the bottom disk is cut by the shield plate, reducing the undesirable disturbance of the servo-head. With this configuration there is no increase in either the weight or the moment of inertia of the head positioner. However, the use of such a shield plate is limited to the situation where only a servo-head in the space formed between two opposing disks. This is one disadvantage of the shield plate proposed by Sengoku.
For a long period, an improved shield means for shielding a servo-head that overcomes the above disadvantages has been sought in the art.